New Construction finished Basement HVAC System

Discussion in 'HVAC Heating & Cooling' started by Joel schoenborn, Jun 14, 2018.

  1. Joel schoenborn

    Joel schoenborn New Member

    Joined:
    Jun 14, 2018
    Location:
    Fairfield, IL
    I am in the process of building a new home in southern IL. 1400 sq ft main level and full finished basement. home is 2x6 construction with dense pack cellulose wall insulation and R49 cellulose ceiling insulation. basement walls are insulated with 1/2" foam board and R11 fiberglass. Windows are triple pane low E. Right now I am leaning toward a 2 stage high efficiency gas furnace and a 2 stage 2 ton A/C. My question is do I need to put the basement on a separate zone to maintain comfort upstairs and downstairs.
     
  2. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    The heat load of a house with those specs is likely to be around/under 15,000 BTU/hr at your 99%outside design temp of about +5F (the design temp for nearby Mt. Vernon IL), which makes just about all 2 stage gas furnaces (except maybe a smaller Dettson Chinook) 2x oversized even at low-fire. The cooling load is probably about 1 ton, 1.25 tons at most.

    Is there any sub-slab insulation in the basement, or is it just the walls?

    The smart thing to do would be to have a competent engineer or RESNET rater run an AGGRESSIVE (per the Manual) room-by-room Manual-J load calculation on the place.

    The first floor can almost certainly be heated and cooled with a fully modulating 1.5 ton Fujitsu 18RLFCD ducted mini-split heat pump. There are other heat pump options that can work almost as well, but most won't be as comfortable as the fully modulating Fujitsu, which can throttle back to 3100 BTU/hr @ 47F, but still delivers over 20,000 BTU/hr @ +17F. (You'd have to consult the extended temperature capacity tables to determine the capacity at +5F, but it's almost certainly going to be enough.)

    A 2-ton Carrier Infinity w/Greenspeed would also cover your load. (Click on the "Heating Capacity" tab, and select the 24VNA024A, in the lower left corner pull down, then try a few different air handler options.) It modulates with a 2.5:1 turn down ratio (drops back to 40% of max capacity), and still delivers ~20,000 BTU/hr @ +5F. It's usually quite a bit more expensive than the 1.5 ton Fujitsu though, and not nearly the same modulation range, but it can drop to under 10,000 BTU/hr when the cooling loads are low. The 1.5 ton Fujitsu would deliver higher comfort since it would run nearly continuously rather than cycling, even in the shoulder seasons when the loads are much lower, since it's minimum modulation is 3100 BTU/hr in both heating and cooling modes.

    The heat loss characteristics of basements are very different from those of the first floor. With the upstairs air-conditioned the only cooling load in the basement will be latent (humidity), so if the main return for the ducted AC takes in basement air, the dry air from upstairs would take care of it. Heating the basement could be a small hydronic (pumped hot water) loop running off the water heater.

    An alternative solution to a heat pump would be an air hander with the right sized cooling coil (probably 1.5 tons or smaller) and a hydronic heating coil running off a condensing water heater. FirstCo has a number air handlers to choose from that would fill the bill. Something like the 24CLX-HW can work with a 1.5 ton condenser, and delivers ~25,000 BTU/hr at an entering water temperature of 130F, and would cover your load even with 120F water. That might actually be oversized, but there are dozens of other models- the 18HBQBR might be a better fit for your loads, but until somebody runs the numbers it's hard to be too specific.

    If you want to take a stab at your own load calculations, loadcalc is easy to use for a quick & dirty first-cut estimate. It regularly oversizes things, so be sure to select for maximum air-tightness and be a bit aggressive on the window specs, etc. Even then it's likely to deliver numbers 15-25% bigger than a more rigorous & aggressive Manual-J using professional tools.
     
    Last edited: Jun 14, 2018
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  4. Joel schoenborn

    Joel schoenborn New Member

    Joined:
    Jun 14, 2018
    Location:
    Fairfield, IL
    Dana thank you very much for taking time to read my post and reply. I did use loadcalc.net to run a load calculation. It had my heating load around 35 ,000 to 40,000 btu's. and cooling load around 20,000 to 24,000 depending on fresh air ventilation cfm's. My basement floor slab is not insulated. I also have to bedroom in basement with 4'x4' egress windows, they are lower quality dual pane with a u-value of around 0.40 I believe.
     
  5. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    I am highly skeptical of the 35-40K heat load for a house that size. My 2400' + 1500' insulated circa 1923sub-code 2x4 framed house with antique clear-glass single panes + clear glass storm comes in under 40,000 BTU/hr @ +5F. I haven't used loadcalc numbers on my whole house, but on one of my additions it ended up a bit more than 35% higher than what the pro-tool Wrightsoft delivered with more aggressive and better specifics on the input options. The error seemed to be primarily the windows loss figures. YMMV. I've seen houses twice your size come in at 40K at lower R values and with crummier windows. Something isn't right.

    The ~2-tons of cooling is also on the very high side for a house with R49 in the attic and triple pane windows. An home energy & HVAC consultant in the Atlanta GA area that regularly performs Manual-J load calculations made this graphic, plotting square feet per ton of cooling against house size. Most of those houses are in the sweaty southeastern US, with higher 1% outside design temps and higher latent loads than in IL.

    [​IMG]

    A cooling load of 20,000 BTU/hr for a 1400' house is 840 square feet per ton which would put your house in the higher load range for houses under 2000 square feet. While that's possible with a HUGE amount of west facing glass, it's not very likely based on the general description of the house.

    If the house is still in the design phase (or can still be tweaked) download a free copy of BeOpt and simulate the house. Sometimes fairly minor tweaks in the window specifications (size and U-factor, SGHC) based on cardinal orientation can make a big difference in peak and average loads.

    A U0.40 window doesn't even meet IRC code max U0.35 for US climate zone 4 (Wayne County is in zone 4).

    The uninsulated slab in the basement is probably worth 6000-8000 BTU/hr of heat loss if it's bare concrete or tile at your ~55-58F deep subsoil temperatures. Did loadcalc separate out the slab loss number?

    If the basement has something other than tile for flooring, note that wood or carpet will grow mold on the underside unless you have active dehumidification in summer. Even an inch of sub-slab (or above slab) continuous EPS reduces the "musty basement smell" risk considerably, and will knock at least something off the slab losses. Two inches of EPS (~R8) is still financially rational on a lifecycle basis, if the slab isn't poured yet. See Table 2, p10 of this document.
     
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